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Internal gain in Er-doped As2S3 chalcogenide planar waveguides Kunlun Yan,* Khu Vu, and Steve Madden Centre for Ultrahigh Bandwidth Devices for Optical Systems, Laser Physics Centre, Research School of Physics and Engineering, The Australian National University, Canberra ACT 2600, Australia *Corresponding author:
[email protected] Received December 15, 2014; revised January 18, 2015; accepted January 22, 2015; posted January 23, 2015 (Doc. ID 230680); published February 24, 2015 Low-loss erbium-doped As2 S3 planar waveguides are fabricated by cothermal evaporation and plasma etching. Internal gain in the telecommunications band is demonstrated for the first time in any chalcogenide glass and additionally in a thin film planar waveguide amplifier configuration. © 2015 Optical Society of America OCIS codes: (160.5690) Rare-earth-doped materials; (310.6860) Thin films, optical properties; (230.7370) Waveguides. http://dx.doi.org/10.1364/OL.40.000796
Rare-earth-doped chalcogenide glasses have attracted considerable interest due to applications in nonlinear integrated optics and, by virtue of their intrinsically low phonon energies, the potential for mid-IR sources at longer wavelengths not reachable with other host glasses. Whilst rare-earth-doped chalcogenide bulk glasses have been intensively investigated, e.g., [1–5], to date, there have been only four demonstrations of gain or lasing in chalcogenide glasses. The first laser action in a rare-earthdoped chalcogenide glass was reported in 1996 where Nd-doped gallium lanthanum sulphide (GLS) glass was pumped to obtain lasing at 1075 nm [6]. This group went on to demonstrate lasing in a 22-mm-long section of GLS fibre at the same wavelength in 1997 [7]. In 2000, optical amplification at 1.34 μm was achieved in a single-mode Pr3 -doped Ga-Na-S (GNS) fiber [8]. Last, lasing in a laser inscribed channel waveguide in Nd-doped GLS bulk glass at 1075 nm was demonstrated in 2002 [9]. Despite the many rare earth elements investigated in bulk glass and the rich vein of possible transitions at longer wavelengths, there have since been no further demonstrations of amplifiers or lasers. In nonbulk glass-based planar waveguide configurations, there has been a quantity of work using erbiumdoped thin films [10–13], and a smaller number of waveguide “amplifier” demonstrations again targeting erbium [14–16]. To date, none of these had progressed beyond limited bleaching of the Er absorption below the internal transparency threshold. In the interests of clarity, we note that the raw internal gain of a waveguide amplifier is considered to be the enhancement factor divided by rare-earth absorption, where the enhancement factor is the ratio of the output power with pump on minus the amplified spontaneous emission to the output power with the pump off [17]. This is the definition used later in this report to measure the internal gain, and represents actual signal amplification down the waveguide length compared to the signal input if waveguide propagation losses are also accounted for. Raw transparency occurs when the internal gain equals 1, and gain exists when it exceeds unity. Net internal gain results when the division by the waveguide propagation losses leaves a number above unity and means more power actually exits the waveguide than entered it. Of the demonstrations referred to above, that of Frantz et al. 0146-9592/15/050796-04$15.00/0
[14] has achieved the best results, quoting an enhancement factor of 2.8 dB∕cm versus an erbium absorption of 5.4 dB∕cm, these quantities measured beyond the Er absorption peak at 1550 nm. Hence the demonstration of Frantz et al. is well below even transparency. As2 S3 has become the workhorse material for studies into planar waveguide nonlinear optics, e.g., [18,19], and also has the advantage for rare- earth-doped applications of a band gap in the green, meaning all tail absorption processes are low at common rare-earth pump wavelengths (e.g., 808, 980, 1064 nm). However it was widely thought that bulk As2 S3 glass cannot be doped without clustering at high enough concentrations for planar waveguide amplifiers [20,21] to date blunting the advantages offered by the material. Thin films however offer an alternative perspective as they are formed in highly nonequilibrium conditions where there is insufficient time and thermal energy for clusters to form, leading to new opportunities for doped devices. However, only a very few works have investigated rare earth (and particularly erbium)-doped As2 S3 films and waveguides [10,22,23]. Fick et al. [10] researched Er-doped As2 S3 films formed by thermal evaporation and subsequent ion implantation. Emission cross-sections up to 1.6 × 10−20 cm2 were measured for the Er3 4 I13∕2 energy level at 1.54 μm, and a 2.3 ms lifetime under 983 nm pumping was achieved for samples thermally annealed postimplant with ∼5 × 1018 cm−3 erbium ion concentration. Improvements in PL intensity of up to almost 10× were seen in some cases after annealing the films just below the glass transition temperature at 165°C for 2 h. Lifetime was also improved by the post-implant anneal but only by about 10%. Highly erbium-doped films (∼4%) were produced by co-evaporation of As2 S3 and Er2 S3 by Lyubin et al. [22]. Emission at 1.54 μm about 3× that of similarly doped GeGaS bulk glass was measured in the obtained films, but no information was presented about the detailed PL properties. Fuchs et al. [23] studied the spectral properties of Er3 -doped As2 S3 deposited by RF sputtering from As2 S3 glass with Er metal partly covering the glass target. PL emission at 1.55 μm displayed a 4-ms lifetime when pumped at 977 nm after thermal annealing at 150°C. The annealing also led to an increase in the PL intensity of up to 40× when measured in a slab waveguide geometry. Erbium concentrations were © 2015 Optical Society of America
March 1, 2015 / Vol. 40, No. 5 / OPTICS LETTERS
estimated at ∼1 mol. % for the highest doped sample, which had inferior PL performance to the lower doped samples with unknown concentrations. Despite these promising results, there is however no report of amplification in erbium-doped chalcogenide devices. In this Letter, erbium-doped As2 S3 thin films and waveguides fabricated through co-thermal evaporation and plasma etching are presented. PL properties of erbium in these films were studied, and the performance of high-quality rib waveguides are discussed from which the first demonstration of internal gain arises. Erbium-doped As2 S3 films were deposited by co-thermal evaporation where separate erbium metal and As2 S3 glass evaporation sources were employed. Evaporation was performed in high vacuum at around ∼10−7 Torr with the thermally oxidized (2-μm-thick oxide) 100-mm diameter Silicon wafers at room temperature and at a throw distance of 40 cm from the sources. The process was monitored by Quartz microbalance thickness monitors, one for each evaporation source and one for the wafers. The evaporation temperature of each source was increased step by step to achieve the desired evaporation rate, which in turn determines the final composition of the film. After the desired evaporation rates were achieved, the shutter covering the wafers was opened, and the evaporation commenced. After deposition, the wafers were thermally annealed in a vacuum oven at 130°C for 24 h to bring the As2 S3 film back closer to the bulk state [24]. The film thickness and refractive index were then measured using a spectroscopic reflectometer (SCI FilmTek 4000) to be 1.35 μm and 2.41 at 1550 nm, respectively. The PL lifetime of the 4 I13∕2 excited state with 1490-nm pumping was measured using an all-fiber confocal set up as previously described [17]. The PL intensity was recorded as well as the lifetime. Plasma etching is known to be an excellent means of fabricating low-loss chalcogenide waveguides [25], but the much less volatile erbium compounds formed with chemistries that are effective on the chalcogenide hosts make erbium-doped glasses difficult to etch. The involatile Er etch products tend to form micro-masks that then result in a very rough etched surface. To avoid this issue, strip loaded waveguides were designed comprising an Er-doped As2 S3 slab and an undoped As2 S3 loading strip with 1.35-μm total thickness, 0.5-μm rib depth and 2-μm width. These provided good mode confinement and high mode overlap with the Er-doped area. The overlaps for both TE and TM fundamental modes and the Er-doped area were around 69% thereby promising effective use of both the pump energy and the excited erbium ions. The cross-section of the designed structure and the simulated TE fundamental mode for a 2-μm-wide waveguide are shown in Fig. 1. Bilayer films to match these thicknesses were deposited using the process described above with the 0.55-μmthick pure As2 S3 layer deposited seamlessly by stopping the Er source at the appropriate point during deposition. The ridges were structured using contact lithography with standard positive photoresist and reactive ion etching (RIE) with CHF3 gas [25]. Initially, a film stack with 0.6-mol. % erbium (1.8 × 1020 ∕cm3 ) was deposited as this erbium concentration would provide gain per unit length >5 dB∕cm if fully
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Fig. 1. (a) Cross-section and details of the designed structure and (b) simulated TE fundamental mode for the 2-μm waveguide.
inverted, certainly enough for planar waveguide applications. The PL intensity and lifetime of the film were then measured through the film edge using an all-fiber confocal set up and methodology as previously described [17]. The insertion loss spectrum of a 2.8-cm-long waveguide was also measured by coupling a heavily attenuated supercontinuum source (< − 30 dBm∕nm to avoid bleaching the erbium absorption) into the waveguides through a lensed fiber. The transmission spectrum was recorded by an optical spectrum analyser (Ando AQ 6317). All the results are shown in Fig. 2. Here the 1∕e lifetime shown in Fig. 2(a) is considerably shorter than that in previous reports, some of this being the choice of pump wavelength [26], but the lifetime also shows significant dependence on the pump power indicating ion–ion energy exchange effects are occurring. The PL measurement of Fig. 2(b) shows a response similar to TeO2 material of approximately comparable doping used for reference, but with PL intensity about 3× lower. The waveguide loss spectrum of Fig. 2(c) indicates an estimated propagation loss of about 0.7 dB∕cm at 1550 nm after correcting for coupling losses (calculated at 2.35 dB/facet for reflection and mode overlap based on a finite difference based method [27]), and shows an excellent fit to a 1∕λ4 curve. This indicates that the loss is dominated by Rayleigh scattering off nanoscale inhomogeneities and is unusual for As2 S3 waveguides, which typically show loss curves dominated
Fig. 2. (a) 1/e lifetime (b) PL intensity, both versus pump intensity at 1490 nm, (c) waveguide insertion loss spectrum, and (d) extracted erbium absorption spectrum compared to that in TeO2 .
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by 1∕λ2 responses from sidewall scattering. From the fit, the erbium absorption was extracted and is plotted normalized compared to that for erbium in TeO2 in Fig. 2(d). One immediately striking difference is in the short wavelength absorption edge, where with As2 S3 as a host, the absorption at usual erbium pump wavelengths in the 1480–1490-nm range is considerably weaker than in TeO2 . This suggests that a pump wavelength in the 1505–1510-nm range may be required for efficient pumping. It might be expected that this could be part of the reason for the lower PL intensity observed. The wavelength of the maximum absorption is also red shifted as expected in the higher index host. Given the expected low solubility of erbium in bulk As2 S3 [20,21], the question naturally arose as to whether the annealing process used to restore the film toward bulk state had caused erbium precipitation. Thus films were annealed at different temperatures, and one film was also measured as deposited and then subjected to the standard 130°C 24 h anneal and remeasured for comparison. Figure 3 presents the results. There are two interesting features from this data. First, it is apparent that as the anneal temperature rises, the lifetime of the active erbium ions is increasing significantly. This indicates that the erbium is not optimally incorporated into the glass matrix after deposition or the standard anneal. Second the PL intensity drops markedly as the anneal temperature exceeds ∼130°C and drops rapidly to zero as the glass transition temperature (∼180°C) is exceeded. For reference purposes, the sample measured before and after the standard 130°C 24 h anneal exhibited about a 30% reduction in PL intensity post-anneal at the 1.8 × 1020 ∕cm3 doping used. It is not clear how much of the drop in PL from the standard anneal is attributable to erbium diffusion and clustering as opposed to changes induced by the large structural rearrangement of the host glass, but the reduction is tolerable given the known benefits to the glass stability and properties [24]. The waveguides were next bidirectionally pumped with 300-mW 1490-nm laser diodes, and also subsequently with 200-mW 1505-nm pumps that, as noted, are much deeper into the erbium absorption band. Very minimal bleaching of the erbium absorption was observed (
